This invention generally relates to inching valves for modulating hydraulic fluid to a hydraulic clutch of an automatic or powershift transmission, and more particularly to a pressure limiting arrangement to prevent the operator from modulating such hydraulic fluid above a predetermined partial clutch engagement pressure.
It is frequently desireable to operate the engine of a work vehicle, such as a lift truck or the like, at a relatively high speed to provide the power necessary for the operation of an implement, such as a lift mast, while moving the vehicle at a relatively slow speed, commonly called inching. This provides the operator with precise control over the vehicle to permit, for example, the critical maneuvering necessary for picking up or discharging a load. In vehicles with automatic or powershift transmissions, this is normally accomplished by providing a hydraulic clutch controlled by an inching valve. Such valve is normally actuated by a foot pedal so that by depressing the pedal, the hydraulic pressure necessary to keep the clutch fully engaged is interrupted, thereby allowing the clutch to slip.
Typically, inching valves can control the fluid pressure to the clutch to either completely disengage the clutch or effect its partial engagement. Thus, the valve controls the pressure of the fluid to the clutch from a substantially zero pressure to its full engagement pressure. However, most clutches are not designed to withstand being slipped for any appreciable length of time above a predetermined partial engagement pressure which may be substantially below its full engagement pressure. As a consequence, the prior inching valves have been adapted to permit the operator to freely modulate clutch pressure within a predetermined low pressure range, but to shift abruptly to full clutch actuation pressure whenever such low pressure range is exceeded.
Unfortunately, this abrupt modulation of pressure still requires a small, but definite amount of spool travel, which has, in the past, enabled a skilled operator to modulate the pressure to the clutch above this desired limit, ultimately resulting in a premature failure of the transmission.